Quaternary Neptunium Compounds: Syntheses and Characterization of KCuNpS₃, RbCuNpS₃, CsCuNpS₃, KAgNpS₃, and CsAgNpS₃

The five quaternary neptunium compounds KCuNpS3, RbCuNpS3, CsCuNpS3, KAgNpS3, and CsAgNpS3 (AMNpS3) have been synthesized by the reaction of Np, Cu or Ag, S, and K2S or Rb2S3 or Cs2S3 at 793 K (Rb) or 873 K. These isostructural compounds crystallize as black rectangular plates in the KCuZrS3 structure type in space group Cmcm of the orthorhombic system. The structure comprises MS4 (M = Cu or Ag) tetrahedra and NpS6 octahedra that edge share to form ∞2[MNpS3−] layers. These layers are separated by the alkali-metal cations. The Np−S bond lengths vary from 2.681(2) to 2.754(1) Å. When compared to the corresponding isostructural Th and U compounds these bond distances obey the expected actinide contraction. As the structure contains no S−S bonds, formal oxidation states of +1/+1/+4/−2 may be assigned to A/M/Np/S, respectively. From these results a value of 2.57 for the bond-valence parameter r0 for Np4+−S2− has been derived and applied to the estimation of the formal oxidation states of Np in the binary NpxSy compounds whose structures are known.